1. Field of the Invention
The present invention relates to novel highly water-absorptive polymer particles (hereinafter sometimes simply referred to as "highly water-absorptive polymer"), and to a process for producing the same. More specifically, the present invention relates to a non-spherical highly water-absorptive polymer having an average particle size and a bulk density both in specific ranges, and a narrow particle size distribution, which is excellent in the speed of water absorption and in the retention of water-absorbed gel, and is useful for sanitary goods; and to a process for producing such highly water-absorptive polymer, in which reverse-phase suspension polymerization is carried out in a specific manner.
2. Background Art
In recent years, highly water-absorptive polymers have come to be used not only for sanitary goods such as paper diapers and sanitary napkins, but also in the industrial fields of water-stopping materials, dew-condensation-preventing materials, freshness-retaining materials, desolvating/dehydrating materials and the like, for afforestation, and for agriculture and horticulture; and a variety of highly water-absorptive polymers have been proposed so far, such as hydrolysates of starch-acrylonitrile graft copolymers, crosslinked carboxymethyl-cellulose, crosslinked polyacrylic acids (salts), acrylic acid (salt)-vinyl alcohol copolymers, crosslinked polyethylene oxides, and the like.
As a method for producing a highly water-absorptive polymer by the polymerization of a water-soluble polymerizable monomer, there is known a reverse-phase suspension polymerization method in which polymerization is carried out by suspending/dispersing a water-soluble polymerizable monomer in a hydrophobic organic solvent, and also an aqueous solution polymerization method. The particle characteristics, such as the shape and size of particles, of a highly water-absorptive polymer are greatly dependent upon the polymerization method employed.
In the case of aqueous solution polymerization, it is necessary to crush the polymer produced. The crushed polymer generally has a wide particle size distribution. Accordingly, it is usually necessary to take measures for preventing dusts of fine particles. Further, coarse particles must be re-crushed.
In reverse-phase suspension polymerization, polymerization is carried out by dispersing an aqueous solution of a water-soluble polymerizable monomer in a hydrophobic organic solvent, so that a dispersant is usually used. The dispersant can have a great influence on the particle characteristics, such as particle size and particle size distribution, of the resulting highly water-absorptive polymer, on the attandant properties such as water absorption properties and the gel retention, and on the stable industrial production of the highly water-absorptive polymer.
Thus, when a fatty acid ester of sorbitan, which has conventionally been employed in reverse-phase suspension polymerization, is used as a dispersant (Japanese Patent Publication No. 30710/1979), the resulting highly water-absorptive polymer has a small average particle size of not greater than 100 .mu.m, and, in addition, a considerably wide particle size distribution. It is therefore necessary to take measures for preventing dusts upon handling of the polymer. Moreover, the polymer tends to be poor in the speed of water absorption and in the gel retention. For this reason, various methods have been proposed in order to increase the average particle size.
A typical method for increasing the average particle size is to add a thickening agent to the monomer phase (Japanese Patent Laid-Open Publication No. 191604/1990). This method, however, has the drawback that since the viscosity of the monomer phase is increased, the polymer particles tend to adhere to one another to entirely or partially form lumps.
Besides the above-described method, there have been proposed methods which utilize various other surface active agents for the purpose of increasing the average particle size.
For example, there has been proposed a method in which an oil-soluble cellulose ester or cellulose ether is used (Japanese Patent Publication No. 17482/1989, and Japanese Patent Laid-Open Publication No. 158210/1982). A polymer having an increased average particle size can be obtained by this method. However, the remaining dispersant melts while the polymer is dried, so that the polymer tends to agglomerate, or to adhere to reactor walls. For this reason, when industrial productivity is taken into consideration, it cannot be said that this method is advantageous.
When a monoalkyl phosphate having a linear alkyl group containing 12 to 24 carbon atoms is used (Japanese Patent Laid-Open Publication No. 209201/1986), a polymer having a large particle size can be obtained. However, this surface active agent is not highly soluble in an organic solvent, so that the allowable range of production conditions under which a polymer is stably produced is extremely narrow. When the polymerization is carried out under conditions not within an allowable range, bulk polymerization tends to take place. Therefore, this method is also disadvantageous when stable industrial production is taken into consideration.
A polyoxyethylene alkyl ether phosphate is highly soluble in an organic solvent, and the use thereof provides a polymer having a large particle size (Japanese Patent Laid-Open Publication No. 36304/1992). However, due to the use of one-feed polymerization, the allowable range of production conditions under which a polymer is stably produced is very narrow. When polymerization is carried out under conditions outside this range, bulk polymerization tends to take place. For this reason, this method is also disadvantageous from the viewpoint of stable industrial production.
Further, when a polyoxyethylene alkyl ether sulfate is used (Japanese Patent Laid-Open Publication No. 93008/1994), a polymer having a large particle size can surely be obtained. However, this dispersant is highly hydrophilic, so that the polymer remarkably adheres to the reactor wall during polymerization and dehydration. Therefore, this method is also disadvantageous when stable industrial production is taken into consideration.
As described above, the conventional highly water-absorptive polymers, produced either by aqueous solution polymerization or by reverse-phase suspension polymerization, have the aforementioned drawbacks in the particle characteristics, the performance upon use, and the industrial productivity.
The particle characteristics, e.g. shape of particle, of a highly water-absorptive polymer is closely related to the retention of the water-swollen gel of the polymer in a sanitary article such as a sanitary napkin or diaper, as described below.
A highly water absorptive polymer is incorporated as an absorbent in a sanitary article generally in the following monomer: a polymer is either sprinkled between cottony pulp layers or fully mixed with a cottony pulp; the resultant composite is placed on a substrate; and then wrapped with an air-permeable sheet to prepare the article.
In such an article, the use of the above-described crushed highly water-absorptive polymer with sharp edges and a wide particle size distribution, obtained via the aqueous solution polymerization, entails the following problems: those particles having large particle sizes break through the air-permeable sheet with their edges, and are forced out from the broken parts of the air-permeable sheet; and those particles having small particle sizes, or water-absorbed gels migrate in, or fall off the cottony pulp through the interstices therein.
In the case where a highly water-absorptive polymer obtained by the reverse-phase suspension method is used, if it is spherical and has a small average particle size, the polymer dispersed in the cottony pulp readily migrates or falls off. Even when the polymer has a large average particle size, there is still such a problem if the particle size distribution is wide that those polymer particles having small particle sizes migrate or fall off.
As described above, those highly water-absorptive polymers which are commonly used now are not satisfactory in the shape, size and size distribution of the particles, and in the performance upon use in sanitary goods.